The present invention relates to a method of thermally treating at least one layer, preferably comprising compound semiconductors, for activating foreign or impurity atoms passivated in the layer by hydrogen, according to which at least one layer is heated for a first time interval of less than 120 seconds to a temperature greater than a first temperature at which the specific layer or sheet resistance decreases.
The described method is used for the electrical activation of the p-doping of II-VI and III-V semiconductors that are produced by CVD (Chemical Vapor Deposition), especially by MOCVD (Metal Organic Chemical Vapor Deposition), processes. Such semiconductors are used, for example, for the manufacture of opto-electronic components (e.g. light emitting components such as blue light diodes or laser diodes). During the CVD processes, in addition to the acceptor hydrogen is also incorporated into the semiconductor layer during the p-doping (e.g. in III-V semiconductors Mg, C, Zn, Be, Cd, Ca, Ba, or in II-VI semiconductors N). Along with the acceptor atoms, the hydrogen form an electrically inactive complex, which leads to a passivation of the acceptor atoms (e.g. Mg) and hence to a high sheet resistance. A number of methods are known for activating the passivated acceptor atoms, according to which the electrically inactive hydrogen-acceptor complex (or in general hydrogen-foreign atom complex) are broken up and the hydrogen is removed from the p-doped or implanted layer by diffusion.
A method according to the initially described type for activating the hydrogen acceptor complexes is described in U.S. Pat. No. 5,786,233, whereby during the method the substrate is irradiated with short wave light, the photon energy of which is greater than the energy gap or band gap at the process temperature. The substrates are preferably heated to temperatures of approximately 650xc2x0 C. to 800xc2x0 C. for a duration of two to thirty minutes. In so doing, the complexes are broken up and the previously passivated acceptors (e.g. GaN:Mg) are activated, as a result of which the sheet resistance is reduced by up to several orders of magnitude, and the hole carrier concentration is correspondingly increased, respectively. As a result of the irradiation with short wavelength light, the activation of the acceptors and hence the hole carrier concentration were significantly increased. Since at still higher temperatures, however, the layer is thermally damaged and the p-diffusivity again decreases as the treatment duration increases, pursuant to U.S. Pat. No. 5,786,233 a longer annealing is preferably used at comparatively low temperatures.
Yoichi Kamiura et al.; Jpn.J.Appl.Phys. 37 (1998) L970-L971 describes the influence of UV irradiation upon the Mg activation of GaN films. In this connection, the GaN films are held in a furnace for about one hour at a temperature greater than an activation temperature of about 550xc2x0 C. Upon irradiation of the Mg doped GaN layer with UV radiation, the activation temperature can be reduced approximately by 100xc2x0 C., as a result of which the thermal stress of the substrates is similarly reduced.
In EP 0723303, there is described a light-emitting electronic component that is built up of a hetero structure, and a method for producing the same, according to which at approximately 600xc2x0 C. with the aid of UV laser radiation, an annealing is carried out in order to increase the acceptor activation in the layers and to reduce the sheet resistance, respectively.
In Mamoru Miyachi et al.; Appl.Phys.Lett., Volume 72, No 9, 1101 (1998), there is described the thermal activation of Mg in GaN with the additional generation of charge carriers that are generated by applying a potential to the semiconductor structure. In addition, reference is made to the fact that a p-type or conducting characteristic of GaN that contains Mg is also achieved and can be influenced by an irradiation with low energy electrons.
All of the previously described methods serve for the activation of the hydrogen-passivated acceptors. The drawback of the above described methods is that relatively long process times are necessary for the activation, whereby in general the substrates (e.g. Sapphire, SiC, Si, AIN, ZnO or Al2O3), with the semiconductor films disposed thereon, are subjected to a high thermal stress, and in addition the throughput is very low.
It is an object of the present invention to eliminate these drawbacks.
Pursuant to the present invention, this object is realized in that with the initially described method, within the first time interval at least one layer is heated for a second time interval of up to 60 seconds to a second temperature that is greater than the first temperature, whereby during the method in at least one third time interval charge carriers are produced within the layer by electromagnetic radiation.
By means of the inventive method, the process duration is advantageously considerably shortened for the activation of the hydrogen-passivated foreign atoms (e.g. Mg) in one or more layers (e.g. GaN) that are comprised of compound semiconductors, whereby sheet resistance and hole carrier concentration are comparable with the aforementioned known methods.
The inventive method is preferably carried out in rapid heating or RTP (Rapid Thermal Processing) systems, since in RTP systems the semiconductor can be processed with very precise temperature-time processes and very high uniformity.
The first temperature of the inventive method is selected between 350xc2x0 C. and 900xc2x0 C., whereby for example with Mg-containing GaN (or in general with group III nitrides) a temperature between 350xc2x0 C. and 600xc2x0 C. is preferred. Depending upon the type of semiconductor, the first temperature can also be a function of the selection of the third time interval and of the intensity of the electromagnetic radiation and of the generation of minority charge carriers connected therewith. As the length of the third time interval increases, and with increasing Intensity of the electromagnetic radiation, then depending upon the type of semiconductor the first temperature can be decreased, which advantageously leads to a reduction of the thermal stress of the layer.
The second temperature during the second time interval is preferably selected between 700xc2x0 C. and 1400xc2x0 C. The selection of this temperature depends to a considerable extent upon the material of the compound semiconductor, whereby for example with Mg-containing GaN a second temperature between 850xc2x0 C. and 1200xc2x0 C. is preferably selected. By selecting a higher second temperature, the second time interval can preferably be considerably shortened, which similarly again leads to a reduction of the thermal stressing of the semiconductor layer or of the semiconductor layer system.
Heating the semiconductor to a second temperature that is greater than the decomposition temperature can be effected for a short period of time. If the surface of the semiconductor layer is provided with a coating (e.g. SiO2), or if the semiconductor layer is heated at an overpressure, for example in a hydrogen-free N2 atmosphere, a decomposition of the compound semiconductor takes place only at higher temperatures, as a result of which the second temperature can be further increased. In this connection, time and temperature are selected in such a way that, for example in the case of Mg-doped GaN, the donor centers that result from nitrogen vacancies do not exceed the number of active Mg centers (in general active activator centers), so that in particular a p-type or conducting characteristic of the layer results. As a consequence, there is provided the possibility of setting or establishing the concentration and activation of donor and acceptor centers in wide ranges.
The duration of the third time interval, during which charge carriers are produced within the semiconductor layers by electromagnetic radiation, can be the same as the duration of the first time interval. In this connection, during the overall process portion of the method during which the semiconductor layer has a temperature greater than the first temperature, minority charge carriers are generated.
The third time interval can, however, also be disposed entirely or partially outside of the first time interval. The layer is then irradiated also or only during the heating and/or cooling phase, or another desired temperature-time process step, which is disposed prior to or after the first time interval, within which the layer temperature is still below the first temperature. This is in particular advantageous if a repassivation of activated foreign atom (e.g. Mg) by hydrogen is to be avoided during the cooling, or if as described above the first temperature is already to be achieved earlier, i.e. at a lower temperature.
On the whole, as a result of the position and the length of the second and third time intervals, as well as a result of the first and second temperature, also the spatial concentration profile of the activated and passivated foreign atoms can be set or established. If the second temperature is above the decomposition temperature, the defect concentration and the spatial distribution thereof can also be set or established. Thus, for example, the third time interval can encompass the second, can be the same or within the second time interval, can be disposed prior to the second time interval, can encompass time ranges prior to and beyond the second time interval or time ranges from the second and after the second time interval, or can also encompass time intervals after the second time interval.
The layers can include group III-V and/or group II-VI compound semiconductors, especially group III nitrides such as, for example, GaN.
The energy of the electromagnetic radiation of the inventive method is advantageously greater than the energy gap of at least one layer. As a result, minority carriers are produced within the layer by the electromagnetic radiation, thereby facilitating the activation of the hydrogen-passivated foreign atoms, avoiding a repassivation of the foreign atoms, and supporting the diffusion of the hydrogen.
The inventive method is advantageously carried out, as mentioned above, by means of a rapid heating system (RTP System), since by means of the RTP system defined very short heating processes can be carried out in the range of one second up to 30 minutes. The temperature-time curves of the substrates can be established precisely to the second in a temperature range of room temperature to about 1400xc2x0 C., whereby the substrate can be heated extremely uniformly not only at low temperatures but also at high temperatures. Similarly, in RTP systems various process gases, which surround the substrate, can also be used, whereby the process gas pressure can be adjusted from vacuum conditions to overpressure conditions.